Little Boy was the codename of the atomic bomb that was dropped on Hiroshima on August 6, 1945 by the B-29 Superfortress Enola Gay. It was the first atomic bomb ever used as a weapon, and was dropped three days before the "Fat Man" bomb was used against Nagasaki.

The weapon was developed by the Manhattan Project during World War II. It derived its explosive power from the nuclear fission of uranium 235. The Hiroshima bombing was the second artificial nuclear explosion in history (the first was the "Trinity" test), and it was the first uranium-based detonation. Approximately 600 milligrams of mass were converted into energy. It exploded with a destructive power equivalent to between 13 and 18 kilotons of TNT. Its design was never tested at the Trinity test site, unlike the more complex plutonium bomb (Fat Man), which was tested. The available supply of enriched uranium was very small at that time, and it was felt that the simple design of a uranium "gun" type bomb was so sure to work that there was no need to test it.

Fat Man is the codename for the atomic bomb that was detonated over Nagasaki, Japan, by the United States on August 9, 1945, at 11:02 a.m. (JSP). It was the second of the only two nuclear weapons to be used in warfare and was the third man-made nuclear explosion. The name also refers more generically to the early nuclear weapon designs of U.S. weapons based on the "Fat Man" model. It was an implosion-type weapon with a plutonium core, similar to the Trinity device tested only a month earlier in New Mexico.

"Fat Man" was detonated at an altitude of about 1,800 feet (550 m) over the city, and was dropped from a B-29 bomber Bockscar,. The bomb had a yield of about 21 kilotons of TNT.

"Tsar-bomb" is the nickname for the RDS-202 hydrogen bomb—the largest, most powerful nuclear weapon ever detonated.

Developed by the Soviet Union, the bomb was originally designed to have a yield of about 100 megatons of TNT; however, the bomb yield was reduced by half in order to limit the amount of nuclear fallout that would result. One bomb was built and tested on October 30, 1961, in the Novaya Zemlya archipelago.

Castle Bravo was the code name given to the first U.S. test of a dry fuel thermonuclear hydrogen bomb device, detonated on March 1, 1954, at Bikini Atoll, Marshall Islands, by the United States, as the first test of Operation Castle (a longer series of tests of various devices). Fallout from the detonation—intended to be a secret test—poisoned the islanders who inhabited the test site, as well as the crew of Daigo Fukuryū Maru ("Lucky Dragon No. 5"), a Japanese fishing boat, and created international concern about atmospheric thermonuclear testing.

The bomb used lithium deuteride fuel for the fusion stage, unlike the cryogenic liquid deuterium–tritium used as fuel for the fusion stage of the U.S. experimental Ivy Mike device, which, being the size of a small office building, was an impracticable weapon for use at war. The bomb tested at Castle Bravo was the first practical deliverable fusion bomb in the U.S. arsenal.

Castle Bravo was the most powerful nuclear device ever detonated by the United States, with a yield of 15 Megatons. That yield, far exceeding the expected yield of 4 to 6 megatons, combined with other factors, led to the most significant accidental radiological contamination ever caused by the United States.

In terms of TNT tonnage equivalence, Castle Bravo was about 1,200 times more powerful than the atomic bombs which were dropped on Hiroshima and Nagasaki during World War II. The largest nuclear explosion ever produced was a test conducted by the Soviet Union several years later, the ≈50 MT Tsar Bomba.